You've mentioned 3 keys that you'd like to play with your instrument tuned to DGdd which would give you the key of G with the scale starting on 3 on the melody string. You'd have a D and G drone as you would with the traditional GDD tuning only the drones would be reverse. This would mean using DAA or 1-5-5 tabs.
If you play the middle string as the G on the open string, you could use DAd or 1-5-8 tabs. You wouldn't have the G drone though, only the lower D on bass and higher d on melody.
To play the key of D as DAdd all you need to do is tune the middle string one note higher from G to A. Use DAd tabs.
The Key of A is normally played AEE, AEA on a baritone and uses heavier gauge strings as the A is the A below the C3 or the C below middle C. However, if you are tuned to DGdd you can do a sort of reverse A as EAe by tuning you bass D one note up to E, the middle string G one note up to A, and the melody strings dd one note up to ee. Again you have the E and A drone with the a at 3 on the melody strings so you'd need DAA or 1-5-5 tabs.
If you are a chord player, you'll have to rearrange your fingers for the proper pattern, but if you're a melody drone player, the tunings should work.
I normally tune CGG but frequently go G by going to DGG . I tune my bass up one note and switch from 1-5-5 tab to 1-5-8. Any lower notes are on the bass string.
Try fiddling around with your tunings but remember to always keep the string you're tuning sounding. If it becomes to hard to change the note, stop tuning. The string will be stretched as far as it's limit.
